Defective 2D Research Articles

Disentangling the catalytic origin in defect engineered 2D NiCoMoS@Ni(CN)2 core-shell heterostructure for energy-saving hydrazine-assisted water oxidation

The major hindrance to efficient electrocatalytic hydrogen generation from water electrolysis is the sluggish kinetics with corresponding large overvoltage of oxygen evolution reaction. Herein, we report a defective 2D NiCoMoS@Ni(CN)2 core-shell heterostructure derived from Hofmann-type MOF as an efficient and durable high-performance noble metal-free electrocatalyst for hydrazine oxidation reaction (HzOR) in alkaline media. The sluggish oxygen evolution reaction was replaced with a more thermodynamically favourable HzOR, enabling energy-saving electrochemical hydrogen production with 2D NiCoMoS@Ni(CN)2 acting as a bifunctional electrocatalyst for anodic HzOR and cathodic hydrogen generation. Operating at room temperature, the two-electrode electrolyzer delivers 100 mA cm−2 from a cell voltage of just 257 mV, with strong long-term electrochemical durability and nearly 100% Faradaic efficiency for hydrogen evolution in 1.0 M KOH aqueous solution with 0.5 M hydrazine. The density functional theory (DFT) was employed to investigate the origin of catalytic performance and showed that high vacancy creation within the heterointerface endowed NiCoMoS@Ni(CN)2 with favourable functionalities for excellent catalytic performance.

Ultrathin 2D[sbnd]2D WO3-x-C3N4 heterostructure-based high-efficiency photocatalyst for selective oxidation of toluene

Photocatalytic oxidation of toluene for the organic synthesis of value-added benzaldehyde has attracted great attention in recent years. However, activation of CH bonds and selective production of benzaldehyde under mild green conditions remains a great challenge. Herein, a series of two-dimensional (2D) ultrathin heterostructure-based components (WO3-x-C3N4 nanosheets), fabricated by coupling oxygen-deficient WO3-x with amorphous C3N4 via a simple electrostatic self-assembly method, was demonstrated as high-efficiency photocatalysts to produce benzaldehyde in the presence O2 as oxidant at room temperature. Under optimal conditions, the prepared 10WCN sample (10 wt% WO3-x-C3N4) exhibited improved photocatalytic oxidation ability with a good benzaldehyde selectivity (96 %) and high benzaldehyde yield (2738.6 μmol g−1 h−1) within 6 h, which is 6.6 and 3.3 times than that of pure WO3-x (412.5 μmol g−1 h−1) and C3N4 (835.7 μmol g−1 h−1). The enhanced photocatalytic performance was due to the constructed 2D type II heterojunction and abundant oxygen vacancies (OVs), contributing to the photoinduced carriers' separation and migration efficiently. This work indicates the synergistic effect of defect metal oxides and 2D ultrathin sheet-to-sheet heterojunctions in photocatalytic hydrocarbon conversion.

Strength prediction and design of defective graphene based on machine learning approach

Defects are inevitable in two-dimensional (2D) materials. Thus, the strength prediction and design are crucial for practical application of defective 2D materials. Utilizing a dataset from molecular dynamic (MD) simulations, this study aims to predict, as well as design the strength of defective graphene. Through convolutional neural networks (CNN), the constructed residual network ResNet34 can accurately predict the fracture strength directly from the defect configuration of graphene. Meanwhile, ablation class activation map (Ablation-CAM) further identifies the sensitive domains that dominate the fracture strength, in accordance with the crack initiation regions confirmed by MD simulations and experiments. In particular, a new descriptor named sensitive domain factor (SDF) was developed to characterize the important features of sensitive domains. Furthermore, a genetic algorithm (GA) is then applied to strategically optimize the defect configuration under a given defect density, achieving an ideal configuration with the maximum fracture strength. This work pioneers a machine learning framework for the extraction and optimization of defective features in monolayer graphene, offering a novel approach to design the mechanical properties through defect engineering.

Pristine and defective 2D SiCN substrates as anode materials for sodium-ion batteries

Exploring more high-performance anode materials for sodium-ion batteries (SIBs) can smoothly help to build a more sustainable, cleaner, and reliable energy system. This study systematically investigates the potential of 2D pristine SiCN (p-SiCN) and defective (d-SiCN) substrate as candidate materials for SIBs anodes using first-principles computational method. The computational results reveal that the p-SiCN possesses excellent thermal stability and structural cyclical performance, good conductivity, high theoretical specific capacity (1486 mAh·g−1), low diffusion barrier (0.177 eV), and appropriate open-circuit voltage (0.340 V), which allows the p-SiCN substrate more suitable as a potential anode material for SIBs. Furthermore, by investigating the effects of intentionally designed defects on p-SiCN, it is observed that the adsorption energy and diffusion barrier of the Na atom on the d-SiCN substrate increase. Our work demonstrates that investigating the effect of defects on p-SiCN can better explore the properties of anode materials, thus further optimizing the battery performances.

DFT-Assisted Investigation of the Electric Field and Charge Density Distribution of Pristine and Defective 2D WSe2 by Differential Phase Contrast Imaging.

Most properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic-scale electric field and charge density distribution of WSe2 bi- and trilayers are revealed using an emerging microscopy technique, differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.

Advancing electrochemical nitrogen reduction: Efficacy of two-dimensional SiP layered structures with single-atom transition metal catalysts

Researchers are interested in single-atom catalysts with atomically scattered metals relishing the enhanced electrocatalytic activity for nitrogen reduction and 100 % metal atom utilization. In this paper, we investigated 18 transition metals (TM) spanning 3d to 5d series as efficient nitrogen reduction reaction (NRR) catalysts on defective 2D SiPV layered structures through first-principles calculation. A systematic screening identified Mo@SiPV, Nb@SiPV, Ta@SiPV and W@SiPV as superior, demonstrating enhanced ammonia synthesis with significantly lower limiting potentials (−0.25, −0.45, −0.49 and −0.15 V, respectively), compared to the benchmark −0.87 eV for the defective SiP. In addition, the descriptor ΔG*N was introduced to establish the relationship between the different NRR intermediates, and the volcano plot of the limiting potentials were determined for their potential-determining steps (PDS). Remarkably, the limiting voltage of the NRR possesses a good linear relationship with the active center TM atom Ɛd, which is a reliable descriptor for predicting the limiting voltage. Furthermore, we verified the stability (using Ab Initio Molecular Dynamics − AIMD) and high selectivity (UL(NRR)-UL(HER) > -0.5 V) of these four catalysts in vacuum and solvent environments. This study systematically demonstrates the strong catalytic potential of 2D TM@SiPV(TM = Mo, Nb, Ta, W) single-atom catalysts for nitrogen reduction electrocatalysis.

Decoding the origins of strength anisotropy in two-dimensional materials

Defects are inevitable in two-dimensional (2D) materials, which is widely recognized to affect the strength of 2D materials. It is known the uniaxial tension strength is significantly different along different directions of defective 2D materials. The defect induced strength anisotropy should have equal importance to defect strength, but unfortunately the mechanism and quantitative description of defect induced anisotropic strength are still unknown. In principle, the atomic origin of materials failure can be attributed to bond fracture, especially for 2D materials comprised of covalent bonds. From the viewpoint of bond fracture, the mystery of anisotropic strength of defective 2D materials is explored, that strength-orientation relations are consisted of complex multi-curves and each curve corresponds to the fracture of same bond. By considering the balance between bond stretch resulting from external loading and bond strength, a strength theory is developed, which can describe the anisotropic strength of graphene and h-BN with vast types of defects, like grain boundary, void, crack. This work deepens the understanding of defect induced anisotropic mechanical properties in 2D materials, which may facilitate defect engineering in 2D materials.

Advancements in defect engineering of two-dimensional nanomaterial-based membranes for enhanced gas separation.

The advent of two-dimensional nanomaterials, a revolutionary class of materials, is marked by their atomic-scale thickness, superior aspect ratios, robust mechanical attributes, and exceptional chemical stability. These materials, producible on a large scale, are emerging as the forefront candidates in the domain of membrane-based gas separation. The concept of defect engineering in 2D nanomaterials has introduced a novel approach in their application for membrane separation, offering an effective technique to augment the performance of these membranes. Nonetheless, the development of customized microstructures in gas separation membranes via defect engineering remains nascent. Hence, this review is designed to serve as a comprehensive guide for the application of defect engineering in 2D nanomaterial-based membranes. It delves into the most recent developments in this field, encompassing the synthesis methodologies of defective 2D nanomaterials and the mechanisms underlying gas transport. Special emphasis is placed on the utilization of defect-engineered 2D nanomaterial-based membranes in gas capture applications. Furthermore, the paper encapsulates the burgeoning challenges and prospective advancements in this area. In essence, defect engineering emerges as a promising avenue for enhancing the efficacy of 2D nanomaterial-based membranes in gas separation, offering significant potential for advancements in membrane-based gas separation technologies.

Probing Active Sites on Pristine and Defective MnPX3 (X: S and Se) Monolayers for Electrocatalytic Water Splitting.

The state-of-the-art density functional theory approach was used to study the structural and electronic properties of pristine and defective MnPX3 monolayers as well as their activity toward water and hydrogen evolution reaction (HER) catalytic performance. The adsorption behavior of H2O on a pristine MnPX3 structure is of physisorption nature, whereas the adsorption energy is significantly increased for the defective structures. At the same time, the water dissociation process is more energetically favorable, and the reactivity of MnPX3 is determined by the vacancy configuration. Following Nørskov's approach, the HER catalytic performance is evaluated by calculating the hydrogen adsorption free energy on the respective MnPX3 surface. Our calculation results demonstrate that defective 2D MnPX3 with low coordinated P shows significantly higher HER performance compared to the pristine counterpart.

Structural Transition in (C2H5NH3)3Bi2-xSbxI9:[(Bi/Sb)2I9]3- Dimers to [(Bi/Sb)3I12]3- Trimers to (∞1)[(Bi/Sb)2I93-] 1D Infinite Chains.

Antimony/bismuth-based lead-free hybrid halide defect 2D perovskites have been generating enormous research interest due to their inherent excellent optical properties. Exploration of new phases and understanding of their structural and optoelectronic properties are of paramount importance in the process of developing materials for practical solar cell applications. In this article, we have reported a structural transition from the 0D hexagonal phase containing isolated [M2I9]3- (M = Bi/Sb) units to the 1D orthorhombic phase via a new monoclinic phase with novel isolated trimeric [M3I12]3- units in (C2H5NH3)3Bi2-2xSb2xI9. The hexagonal phase is stable up to 2x = 0.6 in (C2H5NH3)3Bi2-2xSb2xI9. With gradual substitution of Sb, the cation-cation repulsion increases, which destabilizes the [M2I9]3- unit, and hence, the hexagonal phase becomes unstable. At intermediate composition, 2x = 0.8-1.6, a new monoclinic phase (S.G.: C2/m) with the composition (C2H5NH3)2Bi2-2xSb2xI8 is formed, containing isolated [M3I12]3- units. The symmetry reduction resulted in larger distortion, which relaxes the strain and stabilizes the trimeric unit in the intermediate compositions. Finally, at higher Sb compositions (2x = 1.9-2.0), the compounds crystallize in the orthorhombic 1D phase. In all three phases of (C2H5NH3)3Bi2-2xSb2xI9, the cationic ethylammonium units are completely disordered over the whole unit cell. Raman study clearly shows the phase transition in (C2H5NH3)3Bi2-2xSb2xI9 and also the structural distortion in (C2H5NH3)2Bi2-2xSb2xI8. Optical property study shows that all the compounds are of indirect band gap type. Furthermore, PL study shows better emission properties of the 1D orthorhombic Sb compounds as compared to the 0D hexagonal and monoclinic phases.

Atomic-scale engineering of cation vacancies in two-dimensional unilamellar metal oxide nanosheets for electricity generation from water evaporation

Water evaporation-induced electricity generation, as an emerging energy harvesting technology, has recently attracted extensive attention owing to its ability to harvest electricity directly from the natural water evaporation process. However, the crucial issue that has restricted its practical applications is the low power density and conversion efficiency presented thus far, which generally originated from the weak water-solid interaction between evaporation-induced water flow and nanostructured materials. Exploration of a regulatory approach that can enhance water-solid interaction at an atomic level is highly demanded. Herein, we propose the Ti vacancy engineering at the atomic scale in two-dimensional (2D) unilamellar titanium oxide (Ti1−δO2) nanosheets for efficient water evaporation-induced electricity generation. 2D Ti0.87O2 and Ti0.91O2 nanosheets with precisely controlled concentrations of Ti atomic vacancies are rationally designed. The water flow passes through the 2D gallery of nanosheets while the Ti atomic vacancies can effectively enhance the water-solid interaction between Ti1−δO2 nanosheets and water flow during the water evaporation process for electricity generation. As a result, a stable open-circuit voltage of ∼1.32 V for the hydrovoltaic device made of Ti0.87O2 nanosheets can be maintained for more than 250 h, outperforming those for the commercial TiO2 and Ti0.91O2 nanosheets. This work indicates a promising way to utilize defective 2D materials for hydrovoltaic technology.

Platinum-adsorbed defective 2D monolayer boron nitride: a promising electrocatalyst for O2 reduction reaction

Hexagonal boron nitride (hBN) has long been thought to be chemically inert due to its wide bandgap and strong covalent bonds.

Classical molecular dynamics study of structural properties of pristine and defective mono-layer 2D h-BNNSs

The structural, stability and vacancy properties of both defect-free and defected 2D h-BNNSs were studied using the classical molecular dynamics (MD) approach. The calculations were performed in the canonical (NVT Evans) and isothermal-isobaric (NPT Hoover) ensembles using the Tersoff potentials with the Verlet leapfrog algorithm to obtain reliable structural properties and energies for defect-free, boron (B) and nitrogen (N) vacancies. B and N defect energies were calculated relative to the bulk defect-free total energies, and the results suggest that N vacancy is the most stable vacancy as compared to the B vacancy. The radial distribution functions (RDF) and structure factors were used to predict the most probable structural form. Mean square displacement (MSD) suggests that the mobility of B and N atoms in the system increases with an increase in the surface area of the nanosheets. Results obtained were compared with the bulk defect-free h-BNNSs.

Boosting Nitrogen Reduction Activity by Defect Engineering in 2D Iron Monochalcogenides FeX (X=S, Se)

Large‐scale ammonia synthesis under ambient environment is a highly demanding technology which is promising to replace the energy‐consuming Haber–Bosch process. Motivated by the crucial iron–sulfur interaction in nitrogenase, the performance of 2D iron monochalcogenides (FeX, X=S, or Se) toward electrochemical nitrogen reduction reaction (e‐N2RR) by means of density functional theory calculations is explored. It is confirmed that pristine 2D FeX is inert for even N2 adsorption while defective 2D FeX with single‐anion‐point vacancy (VX) demonstrates considerable activity for NRR. The enhancement is attributed to the interaction between defect‐induced states and p‐orbitals of nitrogen, which greatly alters the adsorption behavior of N2 molecule. Meanwhile, the relation between the N2 adsorption free energy and theoretical limiting potential (UL) agrees well with the previous report. Moreover, the 2D nature of FeX provides flexible adsorption sites for both N2 and proton, alleviating the competition between e‐N2RR and hydrogen evolution reaction. As the existence of VS and VSe in tetragonal iron monochalcogenides has been validified by experiments, a facile strategy for designing practical and economical NRR electrocatalysts is provided and might be extended to the study of other species of defects in catalysis.

One and two-dimensional control growth of MoSe2−x nanostructures

Here, we successfully synthesize the 1D nanorods and 2D nanosheets of defective MoSe2 by a hydrothermal method. It is observed that the morphology of MoSe2−x samples is well controlled by the amount of reducing agent added during the synthesis. The SEM and TEM images reveal that the mixed dimensional 1D/2D nanorods/nanosheets are present in a sample prepared with 40 mmol of NaBH4. The individual 1D nanorod and 2D nanosheet morphology was observed for samples prepared with 60 and 80 mmol respectively. The defective 1D nanorods and 2D nanosheets exhibit different crystallographic planes dispute to posing 2 H-hexagonal crystal structure. The samples containing 2D nanosheets exhibit better electrocatalytic activity than 1D nanorods due to the presence of catalytic active edge sites.

Thermo-mechanical response of pristine and defective 2D hexagonal boron oxide

Understanding the fundamental properties in two-dimensional group-III chalcogenides materials and the development of methods to characterize and control their collective behavior is a topical area that attracts enormous interest. In this work, using molecular dynamics simulations, we report on the effect of temperature, crack length and point defects on the tensile properties of the BO monolayer. BO monolayer is found to be more flexible to deform but it is harder to break compared to other two-dimensional hexagonal materials. It is also demonstrated that the tensile properties are very sensitive to temperature along the armchair and zigzag directions. Furthermore, cracks at different lengths considerably impact the fracture behavior of the BO nanosheet, whereas crack elongation has no influence on Young’s modulus. For nano-cracks, the limitation of the relevancy of Griffith’s criterion for the BO sheet, with respect to MD simulations, is highlighted. Mono- and bi-vacancy defects, patterned perpendicular to the loading direction, greatly influence the reduction of the fracture strength. This study illustrates BO monolayer as a potential material for use in stretchable applications and in future thermo-mechanical devices.

Unidirectional charge transport originated from defect boundary on two-dimensional heterostructure

Two-dimensional (2D) materials, whether pristine or defect-engineered, are promised as excellent candidates for the next generation of photodetector applications, due to their outstanding figures on optoelectronics. However, reports are focused on homogeneous materials, rather than inhomogenous ones containing both pristine and defective areas. Herein, we demonstrate the boundary of pristine (SnSe2/graphene) and defective (SnSe2-x/graphene) 2D heterostructure generates the contact potential difference (CPD), leading to the unidirectional transport of photoexcited charges. Based on the defect boundary, we construct the self-powered photodetector with high performance, including a broadband photoresponse (450 nm ∼ 1064 nm), high on/off ratio (up to 104), fast response (20 µs ∼ 60 µs), and high responsivity (0.076 A/W ∼ 0.74 A/W). Moreover, utilizing the highly automated focused ion beam technology, periodic defect areas are patterned on the SnSe2/graphene heterostructure, forming 2D photodetector arrays with performance differences less than ± 3%. This work reveals the mechanism of the unidirectional charge transport at the defect boundary and offers a novel avenue for the further development of cosmically 2D photodetector arrays.

Using Coordination Chemistry Concepts to Unravel Electronic Properties of SACs in Bidimensional Materials

Single atom catalysts (SACs) embedded in bidimensional (2D) materials are commonly viewed as chemically doped 2D systems whose electronic properties can be discussed in terms of solid state physics, where the doping atoms are perturbating agents to the semimetallic (graphene) or insulating (h-BN) character of the 2D material. In this Perspective, we present a different and closer viewpoint, where the SAC electronic properties are interpreted in terms of coordination chemistry concepts, as the transition metal (TM) atom is complexed by a giant macrocycle, which is the defective 2D layer. In the crystal/ligand field theories, both electrostatic and covalent interactions between SAC and the surrounding 2D material are accounted for defining the splitting and mixing of the TM d states. This analysis reveals the key role played by the type of 2D material not only in determining spin and charge of SAC but also in stabilizing several SAC oxidative states via charge delocalization for an enhanced catalytic activity.

Accurate energy prediction of large-scale defective two-dimensional materials via deep learning

Crystal defects play a vital role in physical and chemical properties of two-dimensional (2D) materials. The computational cost for a real defective system with more than thousands of atoms is considerably high. Here, we propose a framework to accurately predict the formation energy of defective 2D materials, graphene and molybdenum disulfide (MoS2), at a large scale, by combining deep learning techniques and density functional calculations. To improve the training performance of deep learning models, a multi-layer structure descriptor using chemical bond parameters is proposed. For the defective graphene (MoS2) over 300 nm2 (650 nm2), the calculated mean absolute error for the formation energy is less than 47 meV (53 meV) per 1000 atoms. This study provides a practical solution for the accurate and rapid description of large-scale defective 2D materials.

Metallic phase enabling MoS2 nanosheets as an efficient sonosensitizer for photothermal-enhanced sonodynamic antibacterial therapy

Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets (e.g., MoS2) with metallic phase (1T or 1T´ phase) have been proven to exhibit superior performances in various applications as compared to their semiconducting 2H-phase counterparts. However, it remains unclear how the crystal phase of 2D TMD nanosheets affects their sonodynamic property. In this work, we report the preparation of MoS2 nanosheets with different phases (metallic 1T/1T´ or semiconducting 2H) and exploration of its crystal-phase effect on photothermal-enhanced sonodynamic antibacterial therapy. Interestingly, the defective 2D MoS2 nanosheets with high-percentage metallic 1T/1T´ phase (denoted as M-MoS2) present much higher activity towards the ultrasound-induced generation of reactive oxygen species (ROS) as compared to the semiconducting 2H-phase MoS2 nanosheets. More interestingly, owing to its metallic phase-enabled strong absorption in the near-infrared-II (NIR-II) regime, the ultrasound-induced ROS generation performance of the M-MoS2 nanosheets can be further enhanced by the photothermal effect under a 1064 nm laser irradiation. Thus, after modifying with polyvinylpyrrolidone, the M-MoS2 nanosheets can be used as an efficient sonosensitizer for photothermal-enhanced sonodynamic bacterial elimination under ultrasound treatment combining with NIR-II laser irradiation. This study demonstrates that metallic MoS2 nanosheets can be used as a promising sonosensitizer for antibacterial therapy, which might be also promising for cancer therapies.Graphical